This invention relates to a fuel cell system and, more particularly, to combustors for heating a fuel processor which generates a H2rich gas for the fuel cell.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cells gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors such as a water/gas shift (WGS) and preferential oxidizer (PROX) reactors are used to produce carbon dioxide (CO2) from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,422 and 08/980,087, filed in November, 1997, and U.S. Ser. No. 09/187,125, filed in November, 1998, and each assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
In such typical fuel cell systems, a compressor or other air source supplies air to a fuel cell stack containing a number of individual fuel cells and to a combustor. The stack uses the air and hydrogen to generate electricity.
The stack, however, requires more air flow than it consumes based on the amperage demand or load. The combustor, on the other hand, needs a precise amount of air to maintain proper operating temperature. The air flow through the combustor must change as the fuel or hydrogen supplied to the combustor changes. However, the fuel processor cannot shutdown in milliseconds, so as the load falls, hydrogen flow to the combustor rises temporarily. Without an air flow changing at the same rate as the fuel hydrogen flow rate, the combustor may quickly overheat.
Efficient operation of a fuel cell system depends, in part, on the ability to effectively control the temperature of the combustor during all load conditions and, in particular, during transient load changes. This is particularly difficult during transient operation of a vehicular fuel cell system wherein the reformate fuel requirements and, thereby, the combustor output requirements vary with the changing loads placed on the fuel cell.
Therefore, it is desirable to provide a method and apparatus for controlling the temperature of a combustor particularly during dynamic fuel cell system operation. It would also be desirable to provide a method and apparatus for controlling combustor temperature under all load conditions by controlling the supply of a reserve quantity of air to the combustor. It would also be desirable to provide a method and apparatus for controlling combustor temperature which can be easily implemented in existing fuel cell systems.
A control method and apparatus for controlling the temperature of a combustor in a fuel cell apparatus in which unused oxygen from the cathode outlet of a fuel cell is supplied to the combustor along with a separate air flow stream from an air source or compressor.
In one aspect of the present invention, the control method comprises the steps of:
connecting a cathode effluent outlet of a fuel cell to the combustor;
connecting an air flow path through the fuel cell and a combustor air flow path in series with a air supply;
supplying a predetermined excess quantity of air from the air supply to the fuel cell and the combustor, which excess quantity of air is in excess of the air consumed by the fuel cell and the combustor during operation;
providing an air flow bypass path in the air flow to the combustor; and
controlling the amount of the excess quantity of air which bypasses the combustor to control the temperature of the combustor under all load conditions.
In another aspect of the invention, the step of supplying the excess quantity of air comprises the step of supplying the excess quantity of air which bypasses the combustor in a first predetermined amount during start-up of the fuel processor and in a second predetermined amount when the fuel processor reaches a nominal run condition. Preferably, the amount of air bypassing the combustor is slowly decreased from the first amount to the second amount during transition of the fuel processor from start up to run.
The step of supplying the excess quantity of air is implemented by connecting a fast response air bypass valve in a parallel air flow path with an air flow inlet path to the combustor.
In another aspect, the air demand supplied by the compressor is the greater of the product of the load air requirement and the cathode lambda, or the product of the combustor air requirement and the air lambda, where the cathode lambda is the ratio of oxygen sent to the fuel cell stack and the combustor to the oxygen consumed by the stack, and the air lambda is the ratio of the total air sent to the combustor and air bypass valve to the air sent to the combustor.
In another aspect of the present invention, the control apparatus includes a cathode effluent outlet of a fuel cell connected to the combustor. An air flow path through the fuel cell and an air flow path through the combustor are connected in series with an air supply. Means are provided for supplying a predetermined excess quantity of air from the air supply to the fuel cell and the combustor, which excess quantity of air is in excess of the air consumed by the fuel cell and the combustor during operation. An air flow bypass path is formed around the combustor. Finally, means are provided for controlling the amount of the predetermined excess quantity of air which bypasses the combustor to control the temperature of the combustor under any load condition.
In a specific aspect of the invention, the controlling means controls the air supply to supply the excess quantity of air which bypasses the combustor in a first amount during startup of the fuel processor and in a second predetermined amount when the fuel processor reaches a nominal run condition. Preferably, the second amount of air is less than the first amount of air. The controlling means also slowly decreases the excess quantity of air bypassing the combustor from the first amount to the second amount during transition of the fuel process from startup to run modes.
In one aspect, the means for supplying the excess quantity of air comprises a fast response air bypass valve connected in a parallel air flow path with the air flow inlet path to the combustor.
The control method and apparatus of the present invention provides unique control over the temperature of the combustor in a fuel cell apparatus under all operating or load conditions including start-up when there is no load on the fuel cell as well as during normal operating or run conditions wherein the load on the fuel cell can change quickly. The inventive control method and apparatus ensures that a buffer or excess supply of air is readily available to the combustor to prevent overheating temperatures in the combustor when more than normal quantities of hydrogen fuel are quickly supplied to the combustor from the fuel cell.
The inventive method and apparatus enables the operating temperature of the combustor to be controlled in an efficient series connected fuel cell and combustor arrangement. In addition, an economical and small size, slow response air source or compressor may be employed since the inventive control method and apparatus ensures that a sufficient quantity of air is present in the fuel cell apparatus to accommodate transient air demands placed on the combustor despite any high rate of change of the fuel quantity supplied to the combustor.